FIG. 1 shows a conventional insulation testing method for a coil of a generator. In FIG. 1 a coil (1) is inserted into a slot (2A) of a core (2). As is apparent from FIG. 2 showing the schematic sectional view of the coil, the coil (1) is composed of a core wire (3) and an insulating member (4) surrounding the core wire (3). In the case where the coil (1) is used for a generator, the insulating member (4) is, in general, fixedly and closely contacted to the core wire (3). Therefore when the coil (1) is applied with striking force by a hammer (5), the coil (1) produces a striked sound "kon, kon" or "karn, karn" in accordance with the amount of the sectional area and the length of the core (1). A tester, using experience and perception obtained on the job, hears the strike sound and estimates the condition of the insulation of the insulating member (4) at each portion thereof.
It is well-known that even an insulating member (4), of the coil (1) which was initially estimated as a good condition is gradually deteriorated in insulation efficiency over years of use. The insulation efficiency of the insulating member (4) is deteriorated because the insulating member (4) covering the core wire (3) is swelled or peeled off.
In this case, upon the application of the striking force to the insulating member (4) by the hammer (5), the insulating member (4) produces a more dull sound such as "boko, boko" than the previously mentioned sounds indicative of good condition.
The tester has estimated hitherto the strike sounds produced for good conditions and inferior conditions by his sense of hearing. However, it is difficult and sometimes misleading to estimate the strike sound at a power-plant or a factory, at which the ambient noise is great. Further if a large striking force is applied to the insulating member (4) by the hammer (5) for the purpose of avoiding the misjudgement of the estimation, the insulation efficiency of the insulating member (4) becomes undesirably deteriorated even when the insulating member (4) is in a good condition. Therefore it is necessary to restrict the amount of strike force due to the hammer (5). Further the standards of estimation for the good or bad conditions is undesirably changed in accordance with the physical condition of the tester during the testing time over the course of a day.